The temperature of SOFC Fuel cell stacks must be at least 600° C., and typically 750-1000° C., for operation. In order to heat the stack, to operating temperature, the present technique is to force heated air over the cathode side of the fuel cell using an air blower. The air is heated using an electric heater or duct burner in the process air flow path between the blower and the stack. To avoid excessive thermal stresses in the fuel cells, the temperature difference between entering air and a measure of stack average temperature is controlled. For tubular SOFC stacks, the maximum temperature difference is approximately 400° C. Due to heater and ducting material limitations, the maximum air temperature feasible is typically 750° C. Thus, as the stack approaches the temperature of incipient operation, the rate of stack temperature increase decreases dramatically since there is little temperature difference between the air supplying heat and the temperature of the stack.
FIG. 1 illustrates the air path for tubular SOFC stacks. Heated air 2 enters the SOFC generator 4 and is channeled into fuel cells 6 via air feed tubes 8 that extend nearly to the fuel cell's closed end. The air exits the air feed tube at the cell closed end and then flows in counter flow in the annular passage between the air feed tube and the cell inner wall. Heat is transferred from the air to the air feed tube and the fuel cell by radiation and convection and from the air feed tube to the fuel cell via radiation. Air exits the fuel cell at its open end entering the combustion zone/recuperator section and passes out of the generator module at the exhaust nozzle 12. Pressurized fuel (natural gas) enters the generator module at 14 and passes through the nozzle of an ejector (jet pump) at 18. The ejector draws spent fuel from the plenum 19 and mixes it with fresh fuel and subsequently forces it through the pre-reformer at 16 and thence to the in-stack reformers at 17. The in-stack reformers are not shown in detail for drawing clarity. Reformed fuel exits the in-stack reformers at the closed end of the cells and then passes over the exterior of the fuel cell tubes 6, through a baffle into the spent fuel plenum 19. From the spent fuel plenum a fraction passes to the ejector 18 and the remainder passes through a baffle into the combustion zone. The in-stack reformers 17 are heated via radiation heat transfer from the fuel cells 6.
While heating the dormant fuel cell stack to operating temperature it should be noted that the recuperative heat transfer occurring in the combustion zone/recuperator between hot incoming air in the air feed tubes and cooler air exiting the cell open ends is deleterious to the stack heating process. Further, it should be noted that the air exhausted from the generator module at 12 will be approximately the same temperature as the fuel cells and that as the fuel cells increase in temperature this exhaust of hot gas represents a significant loss of energy. Some designs use heat exchangers on the exhausted air to try and recoup some of the wasted heat. Fuel cells that use this type of heating can be found in U.S. Pat. No. 6,764,784 by Gillett, et al, which also introduces further improvements such as purge areas.
What is needed is a method and apparatus that will heat a stack faster and with greater efficiency. Other difficulties with the prior art also exist, some of which will be apparent upon further reading.